Progress in the Environmental Health Sciences depends upon three concurrent steps. First, the construction of a comprehensive data base as a source of policy decisions. Second, the development of cross-disciplinary assemblages of investigators possessing the resources critical to feeding such a data base. Third, the mounting of training programs producing a flow of scientists able to respond to contemporary problems. The Environmental Health Sciences Center at the University of Rochester is designed to parallel these three steps. Its special role is to track the manifestations and mechanisms of incipient and delayed toxicity in coordinated programs ranging from human populations to behaving animals to reproductive processes to organ systems to cells, and, finally, to molecules. It studies agents such as heavy metals, organic solvents, and respiratory irritants and cellular poisons exemplified by dioxin and aflatoxin.